In general, increasing the current density in the electrolytic refining of copper anodes results in an increased copper production through-put without the need for the extra capital investment. Steam and manpower requirements per unit production capacity may also be reduced.
However, increasing the current density gives rise to the following adverse effects: (1) power requirement per unit production capacity conversely increases, (2) surface roughness of electrodeposited copper cathodes increases, and (3) the contamination of copper product with harmful impurities such as Sb, As and Bi tends to become more pronounced. Nonetheless, operating the tankhouse at current densities as high as one can go yet remains to be important in increasing the through-put of electrorefined copper cathodes, provided some appropriate technical measure can be taken to eliminate the aforementioned disadvantages.
The optimum current density to give the maximum economy in the electrorefining of copper has been reported to be 836 A/m.sup.2 as obtained using a copper electrolyte containing 0.5 M CuSO.sub.4 and 1.5 M H.sub.2 SO.sub.4 (60.degree. C.). However, actual working current densities employed at most refineries are limited to levels considerably lower than the optimum current density (i.e., 200 to 250 A/m.sup.2), partly because the surface roughness of electrodeposited copper cathodes tends to increase with increasing Au and Ag losses. In addition, most commercial anodes can hardly withstand such high current density and are readily passivated.
It is also true that the current distribution widely spreads in the tankhouse due to plumbing error, irregular electrode spacing and variation in the weight of anodes with time, etc., thus making some area of the tankhouse operate at current densities much higher than the average current loading, which can cause the onset of passivity.
In order to prevent anodes from passivating during electrorefining exercise, there have been suggested the adoption of the following measures: (1) lowering the current density, (2) increasing electrolyte temperature, (3) intensifying electrolyte circulation, and (4) properly selecting composition of the copper electrolyte along with the kind and amount of organic additives to be employed. The industry has long awaited a new method for effectively preventing the passivation of copper anodes.
A major object of the present invention, therefore, is to provide a method for the treatment of copper anodes to be electrorefined which makes it possible for electrolysis to be advantageously carried out at a high current density without passivation and which generates a significant increase in the output of copper without requiring any additional equipment.
Another object of this invention is to provide a method for the treatment of copper anodes to be electrorefined, which makes it possible for electrolysis to be advantageously carried out without passivation even when levels of impurities are extremely high in the copper anode.